LITHOLOGV. 



by him in his report on the geology of Massachusetts. Subsequently, 

 aided by his son (the chief of our survey), the subject was more thor- 

 oughly investigated by him, and it was found that the flattening of pebbles 

 between layers of schistose rocks had taken place at many localities, and 

 that a geological significance was to be attributed to the circumstance. 

 Since that time analogous phenomena have been found all over the 

 world, so that it is no longer a novelty; but it is well understood that 

 pebbles of quartz, or of other substances, may be variously altered in 

 form by the processes of rock metamorphism, which are not of that 

 degree of efficiency that entirely obliterate all signs of the original con- 

 stitution of the sedimentary mass. In limestones, well known fossils, 

 which are flattened and contorted, are found between the strata; in con- 

 glomerates, two pebbles, one of which has forced itself half w^ay through 

 its more yielding neighbor, are sometimes found in juxtaposition ; and 

 in a mixture of fine and coarse material, the finer part may be converted 

 into mica schist, while the pebbles do not enter into the mass, but are 

 softened and flattened into thin discs. These facts are again brought 

 forward by those who think that the stratification of such rocks is par- 

 tially or wholly due to pressure acting at right angles to the plane of 

 the lamination ; and they point very definitely to the circumstance that 

 though, in these rocks now under consideration, the stratification is 

 parallel to the original bedding, yet pressure must be considered as a 

 very efficient agent in inducing a schistose or finely laminated condition. 

 At some places in New Hampshire the strata of the schistose rocks 

 are most curiously broken. On Mt. Pequawket a huge breccia occurs, 

 which is composed of large broken and angular fragments of argillitic 

 mica schist. These fragments lie in all directions, and at all angles with 

 one another, but are firmly consolidated into a compact mass. This is 

 not so difficult to understand, for at some other points the fragments are 

 found cemented together by a felsitic mass, and at others quartz por. 

 phyry constitutes the bulk of the rock, and in it the fragments of schist 

 lie embedded. It therefore appears that the strata of schist were crushed 

 and broken by the force that opened a way for the eruption of the 

 quartz porphyry, which forms a large part of the mountain. But the 

 cause of the broken condition of these schists is not always so plain. 

 In the south-western part of the state, such appearances are not rare. 



