Page:Proceedings of the Royal Society of London Vol 1.djvu/321

Rh which some of them again emerge in considerable masses, at a di- stance from the precipice. The dykes are all constructed of hori- zontal prisms, which form a strong contrast with the vertical pillars of the strata they intersect; and yet, says Dr. Richardson, it is but lately that these singular productions have been noticed; and he states that he himself was the ﬁrst who observed them, and gave an account which was published in the Transactions of the Royal Irish Academy.

For the purpose of giving a more accurate idea of these dykes, Dr. Richardson gives two views of one of them, which he had over- looked at the time that his essay upon that subject was published.

Of the strata which form the fagade of the precipice, the ﬁrst is sixty feet in thickness, and consists of large basaltic pillars very rudely formed, but generally extending from one surface of the stra- tum to the other.

The second consists of a substance red as brick, and about nine feet in thickness. '

The third is above ﬁfty feet thick, and consists of that variety of basalt termed irregular prismatic, resembling in its grain the colum- nar basalt, but differing in its construction; as its prisms are small, not articulated, unequal and irregular in the vertical or inclined po- sition of their axes.

The fourth is about seven feet thick, columnar but not regular, generally appearing white from a covering of Byssus samutilis, which has a singular predilection for this stratum.

The fifth is ochreous, but less red than other strata of similar con- sistence; this is very friable, and is generally covered by a coating of grass.

The sixth is composed of rude massy pillars, very coarsely formed, ten feet long. The transition from these to the seventh Dr. Richard- son compared to a hand held downwards, and dividing into separate ﬁngers; since the rude columns of the sixth appear continued into those of the seventh, without any distinct line of separation, but are there found divided into neat slender pillars ﬁfty-four feet in length.

The eighth stratum is also ﬁfty-four feet in thickness, and consists of an irregular prismatic basalt similar to the third.

The ninth is that stratum which has attracted principal attention from the beautiful assemblage of neat pillars of which it consists, and which at the intersection of this stratum have been so long dis- tinguished by the name of the Giant’s Causeway. This stratum is forty-four feet in thickness; it ﬁrst appears at its opposite extremity in Portmoon Bay; from hence it rises and culminates between Ben- gore and Pleskin, with its lower edge 200 feet above the water: in its descent at Noffer it composes the group of pillars called the Organ; at all points wherever it is accessible it has the same grain, with the same size and neatness of its prisms.

The tenth stratum on which these pillars rest, is red as minium, and makes a conspicuous ﬁgure from the brightness of its colour.