English to Polish Art/Literary History

Tytuł utworu z albumu "Harbour of Tears" grupy Camel. "Slates" w tym kontekście nie dają mi spokoju, ponieważ żadne znane mi znaczenie nie pasuje do kontekstu. Archaizm? Rozumiem tekst, wiem o czym jest album ale cóż za "slates" mógł na początku XX wieku wysyłać biedny irlandzki emigrant z Ameryki swoim bliskim w pozostałym w domu? Listy na tabliczkach łupkowych (ale są też "letters"), pieniądze (ale "slates"?)

Nie wiem czy wolno przytoczyć tutaj cały tekst, ale co tam:

"Send Home The Slates" (Andy Latimer/Susan Hoover)

I work for the Union Pike,
out on the Western Line.
I found Uncle Sean in Denver,
and he and his wife are fine.
They send their best,
and like the rest -
they send home the slates.

It took six months from the Cobh,
another six by land,
but the pay is good,
and as I should
I'll send home the slates.

Back on your feet
now lads,
Our time for writing home has passed.
Six miles to spike today,
that's what we need.

So fast lads
we must advance,
Work to the Gandy dance.
Six days to double pay,
that's our reward -
But not 'til the line goes down.

I'll not send empty letters,
I know you need the rent.
Dad, you deserve a new pair of boots,
I know it's money well spent.
So kind regards,
I'll work hard -
to send home the slates.

PS. Dear Ma,
I send my picture.
Don't let the family forget me...