THREE EPOCHS OF TPJASSIC ERUPTION. 41 1 



Holyoke bed was probably a mile south and east of the present outcrop, 

 along the Ime of later trap intrusions. The Vjeds slightly baked the sand- 

 stones below and are amygdaloidal and ropy-surfaced aljove. They often 

 took up great quantities of the rock over which they flowed, and the fact 

 and direction of flow are shown by the marked difference between those 

 fragments and the subjacent rock. Much sedimentary material is in places 

 kneaded into the surface layers of the trap — either before it became solid 

 or in a Ijreccia layer — and is then carried underneath by the under-rolling 

 of the solid and yet plastic front of the advancing sheet. 



2. A gi-eat core, representing a second epoch of volcanic act^^'^ty, nf)w 

 fonns Little Mountain, which lies between Mount Tom and the river behjw 

 Smiths Feiry, and from it flowed a thin but double sheet south beyond the 

 limit of the State and north at least to the river, a half mile south of the 

 Holyoke gaji. 



3. Immediately following this came an explosive outburst which s[)read 

 tuft' south to Holyoke and east across the whole basin to Belchertown. 

 East of the river this rests on arkose; west, on the upper trap sheet. Its 

 masses are largest (3 feet in length) at Smiths Feny and decrease slowly 

 east and south. 



Tlie results of the last period of volcanic activity appear in a line of 

 crater tlu-oats and short intrusive dikes extending from the river to the east 

 edge of the basin, parallel to and a mile south of the Holvoke range. Two 

 are of very large size and one is a diabase full of quartz and feldspar 

 grains. They make a small angle with the tuff sheet, so that some lie 

 south and some north of it and some penetrate it in whole or part. 



DIABASE DIKES AXD STOCKS IX THE GXEISS EAST OF THE TRLIS. 



A series of small dikes appear in the gneiss ea.st of and a short distance 

 from the sandstones. I do not find reason to consider them continuous over 

 so long a distance north and south as they would appear to be from Perci- 

 val's excellent map of their disti-ibution in Connecticut, nor does any trace 

 of the similar western line of dikes marked by him extend northward into 

 Massachusett.s. 



They are typical diabases, much fresher and of finer gi'ain than the large 

 masses in the 8and.stones, but scarcely offering any appreciable distinction 

 from the finer grades of the latter. On their borders, however, and in small 



