85 



pebbles ; but the great majority are petrosilex. The petrosilex 

 pebbles or fragments are of the same color and general appear- 

 ance as the enclosing rock, but are often larger, apparently, than 

 could be obtained by breaking up any of the lenticular layers now 

 exposed. But this appearance is merely illusory ia many cases 

 at least, resulting from the fragments being looked at flatwise ; 

 and often the unbroken lenticular layers themselves have sharply 

 cut, angular outlines when viewed in this manner. In some por- 

 tions of the rock the pebbles are rare or wanting, while in other 

 parts they are very numerous, "forming a considerable propor- 

 tion of the whole, well-nigh obliterating the schistosity, and 

 giving the rock quite the aspect of a breccia. But, as just 

 explained, this appearance is likely to be more pronounced on 

 a surface parallel with the schistosity ; the pebbles in such a 

 view being partly real and partly lenticular layers seen flat- 

 wise. Occasionally the pebbles are banded or striped, and 

 can then be certainly identified with the enclosing rock. In 

 such cases the banding is very likely to form an angle, some- 

 times a high angle, with that exterior to the pebbles ; and in 

 every instance the bands of the surrounding petrosilex immedi- 

 ately adjacent to the pebbles are deflected so as to conform to 

 their outlines. 



There is but little of the gray, schistose petrosilex in sight. 

 So far as observed, it contains fewer pebbles than the red, and 

 the structure is less disturbed ; but otherwise, save that the base, 

 the lenticular layers and the jaspery patches are diflerent shades 

 of gray and white instead of red, the two rocks appear to be 

 identical. A singular fact observed in connection with these 

 two varieties of petrosilex is that, although occurring in con- 

 tact or nearly so, and both showing considerable disturbance 

 and holding pebbles, neither contains fragments that can be 

 referred to the other ; they seem to be petrologically distinct. 

 On the west side of River Street the striping of the gray petro- 

 silex is nearly wanting, and the rock is decidedly crystalline, 

 holding crystals of feldspar and quartz. 



As already observed, the structure of these striped rocks in 



