how can I read old varnish logs?

[John Cherouvim]

In /var/log/varnish/ I have some old varnish logs, produced by 
varnishlog which, due to logrotated, are now in gz format:

 > -rw-r--r--  1 varnishlog varnish 143068514 Aug 10 23:59 
varnish.log.2017-08-10.gz
 > -rw-r--r--  1 varnishlog varnish 156373518 Aug 11 23:59 
varnish.log.2017-08-11.gz
 > -rw-r--r--  1 varnishlog varnish 134255825 Aug 12 23:59 
varnish.log.2017-08-12.gz
 > -rw-r--r--  1 varnishlog varnish 156992529 Aug 13 23:59 
varnish.log.2017-08-13.gz
 > -rw-r--r--  1 varnishlog varnish 176751837 Aug 14 23:59 
varnish.log.2017-08-14.gz
 > -rw-r--r--  1 varnishlog varnish 155948012 Aug 16 00:01 
varnish.log.2017-08-15.gz
 > -rw-r--r--  1 varnishlog varnish 169977134 Aug 17 00:01 
varnish.log.2017-08-16.gz

I've extracted those in another location and tried to view them via 
head/more but they look binary.

So I then tried opening them with varnishlog using either of the 
following parameters:

 > [-N filename]             VSM filename
 > [-r filename]             Binary file input

But that didn't work giving me:

 > Can't open log - retrying for 5 seconds

and:

 > Can't open log file (Not a VSL file:

Any idea on how can I inspect those historical logs from varnish?

p.s I use varnish-4.1.1