258 McMahon and Hutching s — On Pseudo-SpheruUtes. 



origin. Here and there a very fine-grained lamination is to be 

 observed in the base. The micro-granules are arranged in perfectly 

 parallel straight lines, very close together, and possess a rigidly 

 parallel structure, like the lamination of a very fine-grained sedi- 

 mentary rock, and unlike the foliation of an igneous rock. It seems 

 altogether unlikely that a fine-grained laminated structure, like that 

 seen in the slide referred to, in rigidly parallel lines, could have 

 been produced here and there in isolated patches in a vitreous rock 

 of igneous origin. Their presence, however, in a partially melted 

 fine-grained sedimentary rock is easy of comprehension. 



The pseudo-spherulites in the Dinas Head specimens present, 

 on careful examination, some features different from the normal 

 spherulites of glassy igneous rocks. The pseudo-spherulites do not 

 exhibit a dark cross when revolved between crossed nicols. Indeed, 

 the base of these sperulites, namely, the micro-granular, or crj'pto- 

 crystalline material seen in the centres, and between the blades of 

 the felspar prisms, which radiate from the margins of the spherulites 

 towards their centres, does not possess a fibro-radial structure at all; 

 and does not in any way differ in structure from the crypto- or 

 micro-grfinular base outside the boundaries of the pseudo-spherulites. 



In normal spherulites the radial fibres, or fibrous prisms, have 

 straight extinction ; v/hereas in the Dinas Head pseudo-spherulites 

 the prisms which radiate from the outer edge towards the centre 

 (which they never reach) are those of a triclinic felspar and have, in 

 every case, oblique extinction. 



The conclusion reached by General McMahon regarding the 

 pseudo-spherulites of the Dinas Head soda rocks is, that they are 

 the product of contact action hy a basic trap on a sedimentary rock 

 which either contained, in whole or in part, the chemical constituents 

 of albite ; or into which those constituents were introduced, in 

 whole or in part, at the time the contact action took place. 



As the presence of spherulites in a vitreous rock has been 

 regarded by many petrologists as evidence of the igneous origin 

 of the rock in which they occur ; and as a metamorphic origin has 

 not hitherto, so far as we are aware, been suggested for spherulites, 

 this determination seemed worth noting. 



The independent researches of Mr. Hutchings, in another locality, 

 carried out at the same time, has supplied the needed confirmation. 

 This confirmation is all the more important as no doubt can be 

 entertained regarding the sedimentary origin of the rocks studied 

 by Mr. Hutchings and described in the March number of this 

 Magazine. They are shales forming part of beds of limestone, 

 sandstone, and shales, in contact with the Whin Sill, and still 

 retaining in their altered condition abundance of clastic quartz- 

 grains and other witnesses to their original nature, even if this 

 was not as positively assured as it is by the field evidence. 



In these altered shales Mr. Hutchings describes at pages 124, 125 

 nodules which exhibit under the microscope much the same general 

 character as those of the Cornish' rocks, and which contain in some 

 cases numerous well-developed spherulitic aggregates of fibres of 



