Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/283

 structure which occurs in the former. Where the marble is at the greatest distance from the granite it differs little or nothing in hardness or composition from ordinary specimens of this substance. But wherever it approaches or comes into contact with the granite it becomes highly indurated, effervesces slowly with acids, and gives on analysis a large portion of siliceous matter. In other respects there is no obvious change at the planes of contact, and the union between the granite and limestone is so slight that the action of the water separates them. The union of the limestone with the schist is much more intimate. Wherever the granite traverses both these substances together, which is generally the case where the schist is present, such a confusion takes place, that the boundary of neither can be defined, a troubled mixture and then a complete gradation between these two rocks being the result.

On the right bank of the river above this rock a bed may be seen, consisting of a brownish hornstone, or rather a compact felspar, with a character somewhat porphyritic. It is one of that great variety of rocks associated under the general name of porphyry of which frequent examples occur in the course of the Tilt on both sides of the valley; more than I have thought it necessary to mention. These are sometimes considered as veins, and sometimes as beds; I believe that the former disposition is by far the most common, but their real nature is often so difficult to ascertain that I have been obliged to abandon the point in despair. The present one appears to be a bed.

I have purposely avoided noticing the rocks on both sides of the river which lie beyond its immediate bed, as they offer, on the right side at least, a succession so mixed and so confused, that any view of them which can be obtained in following the stream is nearly