Page:The South Staffordshire Coalfield - Joseph Beete Jukes - 1859.djvu/136

118 sometimes fine-grained or compact, sometimes largely crystalline. It contains sometimes fibrous radiated masses and plates of some zeolitic minerals.

From this Greenstone proceed dykes and veins of "White rock" trap, which at first sight might be mistaken for a compact sandstone, but when closely examined bears a greater resemblance to a partly decomposed, white, compact, and somewhat earthy felstone. In some places little shining facets of feldspar may be detected in it. The colliers all unite in stating that this "white rock" proceeds from the "green rock," and though I have never myself seen the junction of the two rocks. I have no doubt of the fact. That the "white rock" is an igneous rock, as well as the "green rock," is proved by its cutting through the coal and other beds in the same way that the green rock does, and by its producing the same amount of alteration in them.

A specimen of this trap was analysed by Mr. Henry, which gave the following results:—

This composition shows us at once that the rock cannot belong to the siliceous class of the Felstones or Trachytes, but is one of those basic compounds to which the Greenstones, Diorites, and Dolerites belong. Moreover, the presence of so large a percentage of carbonic acid and water shows that the rock has now a different composition from that which it possessed before the period of injection, when it was in a molten condition. It is, in fact, an altered rock, altered by having had so much carbon, oxygen, and hydrogen added to it as to give it a percentage of 20.330 of carbonic acid and water. If we deduct those materials from Mr. Henry's analysis, and reduce the remainder to percentages, we get—