278 W. S. Ba/yley — Soda- Granite, etc., of Pigeon Point 



rocks in the former. In those portions of the contact belt 

 nearest the gabbro (indicated on the map by horizontal lining), 

 more particularly in the western portion of the point, there is 

 a band of breccia of varying width. This consists of a cement 

 of gabbro, or of one of the most basic of the intermediate rocks 

 (a mixture of gabbro and red rock).* holding numerous in- 

 clusions of slate and quartzite. The slate fragments in this 

 breccia have been altered like the slates in the innermost con- 

 tact zone. The fragments of quartzite are fringed by a rim 

 which is exactly like the material of the keratophyre. One 

 fragment that shows this red border most beautifully is a rhom- 

 boidal block of pink quartzite, about seven feet in length and 

 four feet wide. Surrounding it is a bright red border some 

 two and a half or three inches in width. In the thin section 

 the material of this border is observed to have the granophyric 

 structure of the red rock. Fan-like groups of feldspar and 

 quartz extend perpendicularly from the bounding planes of the 

 inclusions. Since this rim is probably the result of the fusion 

 of the peripheral portion of the fragment by the surrounding 

 rock, and its structure and mineralogical composition are iden- 

 tical with these of the red rock, it may fairly be concluded in 

 the absence of any evidence to the contrary, that the red rock 

 itself has been produced in the same way by the fusion of the 

 quartzites (and slates) by the gabbro. 



Moreover, if the red rock is a fused sedimentary, there 

 should be intermediate stages in the transition from a clastic 

 rock to a crystalline one. The red quartzites of the inner con- 

 tact belt have a structure that can be explained by supposing 

 them to be quartzites whose interstitial feldspathic and chlo^ 

 ritic substance has been fused, or by regarding them as fragmen- 

 tal rocks into which the liquid magma of the red rock has been 

 squeezed by pressure. In either case the porphyritic and irreg- 

 ularly outlined quartz crystals so commonly found in these rocks 

 are probably the corroded remains of the original grains of the 

 quartzite. On the top of the porphyry bluff, whose position is 

 indicated on the accompanying map (A), the porphyritic red 

 rock is abundant on the west side of the cliff. As we pro- 

 ceed eastward mottlings are discovered in it that resemble the 

 mottlings characteristic of certain zones of the contact belt. 

 On the extreme eastern side the prevailing rock is an altered 

 red quartzite. No line of demarkation can anywhere be discov- 

 ered between the different types. 



Although these gradations may not serve to prove positively 

 that the quartzites by fusion gradually pass into the red rock, 

 the almost universal presence of transitional phases between 

 the red rock and the altered quartzites in those places where 



* This Journal, Jan., 1889, p. 58. 



