SPHERULITIC VOLCANICS IN MA INK. 475 



Pirsson. Haviii*,' become interestetl in tlie si)ei*iinens .sent me l)y these jjentlemen, 

 a three (hiys' visit was made to North Haven, and from this point as a center a few 

 trips were made by water alon*; the neighboring!; creeks and inU'ts, and a lar;j:(> 

 number of sptrimens were collected. 



Of coni*so, in the short time at my disi)osal, I could do little more than note tlie 

 characteristics of the varieties of rocks observed. Their relations to the surround, 

 ing Paleozoic sediments were not investi^rated, nor was their areal distribution 

 mapped. Knouj^h, however, was seen to convince me that a great (juantity of old 

 lavas and tulls had been spread out over the regit m in early geological time. 



The rocks occur, so far as was determined, mainly in sheets which are nearly 

 horizontal. They comprise dense and porphyritic, dark basaltic looking rocks, 

 amygdaloids, light and dark tulls, breccias, conglomerates and spherulitic ancient 

 glasses. These beds are cut by diabase and basaltic dikes, whose composition in 

 some instances is like that of some of the beds. 



Many of the rocks have been so completely altered that their original composi- 

 tion can no longer be determined. They consist now of (piartz, chalcedony, calcite 

 and occasionally other secondary minerals, so aggregated, however, that they often 

 preserve the original rock structure. From this we learn that some of the lavas 

 were basalts; others were probably rhyolites. Someof the former were i)orpliyritic i 

 the latter were usually glassy. In the porphyrites the plagioclase crystals are still 

 preserved, and these are arranged as in the modern andesites. Sometimes the de- 

 composition products of olivine may be detected in them, so that we may safely 

 regard these porphyrites as basic flows. 



The tulFs associated with the more basic rocks i)resent all the structural features 

 of recent tulfs. Although they have l(jst their original composition, their structure 

 can still be recijgnized as typically tuffaceous. 



Amygdaloids are quite common, especially west of Xorth Haven, on the shore. 

 Their amygdules are now filled with calcite and other secondary minerals, and 

 indeed the rock mass is usually completely altered. There can be no doubt, how- 

 ever, that the rocks are true amygdaloidal flows. At their contacts with other 

 rocks the amygdules are large, and away from the contact walls the beds are not 

 infreijuently (piite free from all traces of the amygdaloidal structure. 



As for the rhyolites, we have little on which to base an argument as to their 

 original condition. Many of them present in the weathered surfaces beautiful ex- 

 amples of flowage lines, sometimes forming a series of parallel striations, but more 

 frei|uently producing patterns of comi)licated designs. One of these rocks was de- 

 scribed by Dr Williams, who thought there was no question but that it was origi- 

 nally a gla.ss. Perlitic cmcks, now filled with calcite and silicii, cross many sections, 

 exhibiting l)eautifully a rudely concentric series of rings. Occasionally the rhyolites 

 contain small i)orphyritic crystiils, but usually they are non-porphyritic. 



The tufls associated with the rhyolites are as characteristi(- as those a.ss(x-iated 

 with the more basic rocks. Their structure, if ncjt their composition, has so fre- 

 quently l)een i)reserved that no one can doubt their true nature. Of course it is 

 impossible to declare positively, without nmch study, that some of the fragments 

 arc gliu^s, but it is the opini<in of tin; si)eaker that they are. 



.V few of the rocks which are provisionally clas.sed with the tuffs appear to cdh- 

 tain water-worn gniins, and some of them seem to be composed mainly of this 

 material. It may be tiiat by further investigation we shall learn whether the old 

 volcano was terrestrial or submarine. 



